devicetree: document Ingenic SoC UART binding

Add binding documentation for the UARTs found in Ingenic SoCs.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: devicetree@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/10161/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>